The research described below forms part of an HCV Cooperative Research Center focused on understanding the biological events that occur during acute HCV infection. It is remarkable that as few as 11 HCV proteins can thwart innate and acquired cellular defenses to promote virus replication and survival. In the current proposal, we will characterize HCV-host interactions involving hepatocytes, natural killer (NK) and dendritic cells (DC) that contribute to the resolution of acute HCV infection. This proposal is intentionally focused on viral determinants that regulate innate immunity during acute infection. Indeed, HCV proteins have been shown to interact with various signal transduction pathways at multiple levels including inhibition of interferon alpha (IFN) signal transduction, direct inhibition of IFN-induced proteins, and blockade of IFN regulatory factors (IRF). In particular, the HCV core protein has many interactions in human cells, and hence is strongly implicated in HCV persistence and pathogenesis. We have recently found that the HCV core protein modulates IRF and IFN pathways. Strikingly, different clinical isolates of HCV core display heterogeneity in interaction with IFN pathways. This proposal will test the hypothesis that differential modulation of the innate signal transduction pathways by HCV core affects the outcome of acute HCV infection. We will also examine extracellular versus intracellular core-host interactions by adding recombinant core protein to cells or expressing core within cells. Aim 1 will generate recombinant core proteins from 50 acutely infected patients, 25 who resolve infection, and 25 who progress to chronicity. Core genes will be sequenced, analyzed by phylogenetics, and analyzed for correlations with outcome of acute infection. The effect of HCV core on human hepatocyte innate antiviral pathways will then be tested. In Aim 2, the effect of core protein on IFN signaling in natural killer (NK) and dendritic cells (DC) in will be evaluated. Aim 3 will determine whether NK and DC antiviral, pro-inflammatory and immunodulatory responses during acute infection and HCV core protein exposure correlate with outcome of acute infection. Understanding how the HCV core protein interacts extracellularly and intracellularly with innate cellular defense pathways is important for controlling acute hepatitis C, for defining viral and host markers of acute infection and outcome, and may afford new opportunities for therapeutic development.